During the course of Star Wars: The Last Jedi, Rey finds an exhiled Luke on Achh-to in order to motivate him to train her and help the Resistance.
At first Luke refuses and he vaguely explains to Rey,
Luke Skywalker: I've seen this raw strength only once before. It didn't scare me enough then. It does now.
The bigger revelation for the rest of film is about discovering a key event in fully turning Ben Solo to the dark side, as Luke nearly caves into killing his nephew, before he stops himself, but in Ben's eyes, it was too late.
It seemed like in the context of the film, given Rey & Kylo Ren's ongoing connection and force bond, that Luke was referring to Ben Solo/Kylo Ren in being the other person with "raw strength".
But in light of the upcoming film, Star Wars: The Rise of the Skywalker and the alleged return of
Darth Sidious/Emperor Palpatine,
I was wondering if Luke was really referring to Ben, or if it was perhaps this other character or someone else entirely?